True sine wave inverters will still be expensive. Your brothers 2000 watt generator isn't likely a very clean source of power and I doubt it's true sine wave. If you don't need true sine wave you can get a 1500Watt (3000 peak) Xantrex inverter on Amazon for the same price...around $115. You can also cover the gap and get a 'modified' sine wave inverter which does a fine job as well.

There should be a name plate on your furnace. It will list the power requirements in either watts or amps.

Home built 12 V generators.
Home Brew Power
My homemade generator "Generax 1500"
Home Made Generator

Hi harstad, I too have been looking at this situation, we get a lot of power outages here in Cairns, cyclones, flooding etc and was tired of being without power for up to a week or so and this is what I did.

1. Figure out what you really want to run and add up all the watts.
2. Buy a generator that will handle the load to run everything including A/C (in my case a 6kva)
3. Figure out your power usage less your A/C and purchase a bank of AGM battries (may be only 2-3) a decent inverter a smart battery charger and a battery monitor, your battery bank should carry you for 3 to 4 days or so before needing to be charged, this will run lighting, fridge, computers, fans etc.
4. Get a sparky to install a power point in your electrical box and a change over switch, this insures no power goes back into the grid when you plug in your generator or running from the battery.

I use the generator as a back up and to charge my batteries and for those realy heavy days to run hot water and aircon thus keeping the cost and noise down running the genset full time. There is no cheap solution to this type of power supply so shop arround. Dont forget to run a generator once a month and put a good load on it and learn to look after those battery's, a lot of people let gensets sit in the garage for years without firing them up and wonder why they dont work!!! dito for the battery's you should get about 8 years from these IF you keep them charged correctly, this will insure a good working power supply when you realy need it most. This back up system did set me back a few K but what the hell when Sh*t did hit the fan my wife,kids and myself sailed through it comfortably.

Harstad and tonyb both provide great information.

At a previous home, we added a sub panel next to our primary electrical panel that would carry the load from our back up generator (B&S 7500w) to several critical loads needed in our house (well pump, refer, propane insert fan, several lights, etc.).

At our current home we added a 3.6kw solar system that we are backing up with an AGM battery bank (three sets of 48v or 12-12v batteries). This will allow our home almost normal use - except for luxury items like the A/C, garbage disposal, dishwasher, W/D. And, we can charge our batteries while providing power to our house with the back up generator that we brought with us. During several days of storms when there is no solar benefit, the back up generator should fully charge the batteries within a couple of hours giving us ample power for overnight without running the generator constantly.

It really depends on how much you want to invest into your home. A good generator with high quality extension cords (don't buy cheap ones as they don't carry loads nearly as well - read their specs! buy cords that will carry construction loads - power tools, etc.) can easily get you through a 72 hour dilemna provided you are conserving. Battery back ups can get expensive, but are a great safeguard whether tied to a solar system or charged by your power provider. Having the ability to charge your batteries during an outage with a generator is a plus too.

Regarding using your A/C during a power outage, I would say forget it! Unless you go crazy with an overkill back-up system you will not be able to carry the load. I've watched our 'net meter' go from selling power to our utility at 2.25kw per hour to buying 4.5kw per hour once the compressor on our A/C kicked on - a 6 to 7 KW swing. You will overload most standard generators or drain your battery back up in minutes if you try to run your A/C.

That's my 2 1/2 cents anyway.

Hi Hastard,

Go to this link and learn the A-Z on backup power, this guy is realy genuine and helps a lot of people, his site is very informative and you will get a lot out of it regarding generators, batteries etc.
Follow the link and go to Tech InfoFridge and Solar

Jumping in a bit late here, sorry for that.

Two thoughts on generators while the power is out:

1) For longer term there is also a psychological factor to those around you. The power went out here for a week at a time twice last year. During the day it wasn't really much to hear the generators running. After dark fell and we'd laid down to bed the only thing you could hear was the two generators in the neighborhood. After a few nights of being kept up by them it was easy to resent them. That's one more advantage of having them set up with batteries. The ability to still provide power to what you need without drawing as much unwanted attention.

2) It's possible to modify a chest freezer into a chest fridge that consumes about 0.1 kWh per day*. Making modifications like that may allow you to use a less expensive setup, run more devices or have more head room.

*http://www.mtbest.net/chest_fridge.html an example of a chest fridge.

The fact that your furnace is on a 15 amp breaker, does not mean that it consumes 15 amps, indeed the NEC requires that circuits supplying continous loads are not loaded to more than 80% capacity, therefore if instaled to code the furnace should draw 12 amps or less, probably a lot less.

The actual draw may be determined from the manufactuerers, or if cord and plug connected, by measurement.

Running whole house or central A/C is not normaly feasible from an an affordable portable generator. Such A/C is generally 208/240 volt and is not cord and plug connected.
A sufficiently large generator could of course run the whole house, including A/C,this would require a changeover panel between the meter and the main breaker panel, unless you have very considerable experience, this should be done by a licensed electrician.
A larger portable generator could be used to power a 120 volt cord and plug connected A/C unit, either of the throuh the window type, or the floor standing style that useing a flexi duct to remove the hot air.